In order to better define the relationship of solute size to its clearance for the native kidney and how it differs from that for hemodialysis, peritoneal dialysis and hemofiltration, the following experiments will be undertaken. Dextrans which have been widely used to characterize the permeability of native and synthetic membranes will be used. A family of dextran fractions with narrow size distribution and of ascending molecular weight (1000 to 40,000 daltons) will be generated and used to develop and compare the clearance vs. size relationship of the glomerular basement membrane of the dog and sheep with that of the simultaneously measured clearance vs. size relationships of the artificial kidney or the peritoneal membrane. Clear understanding of the differences in these excretory patterns will shed light on what size fractions of uremic plasma to examine in order to determine what solutes are responsible for clinical differences noted between treatment modalities and why "well" dialyzed patients with chronic renal failure show continuing serious abnormalities.